Because they are B-stageable and have very good properties, epoxy resins are widely used in making laminates. Laminates have also been made using polyimide-based laminating resins, which have many high performance characteristics not possessed by epoxy resins. However, polyimide resins require more severe curing conditions than do epoxy resins to achieve the optimum properties.
Another approach to improving laminates has been to modify the epoxy resins with imide functional adjuvants. For example, U.S. Pat. No. 4,244,857 teaches the use of certain bis amino imides as curing agents for polyepoxide resins. Similarly, U.S. Pat. No. 3,978,152 discloses thermosetting compositions which are blends of certain unsaturated bisimides with adducts possessing an amino group formed from an epoxy resin and excess amine. U.S. Pat. No. 3,984,373 describes a thermosetting resin composition based on an epoxy resin incorporated with an N,N'-unsaturated amic acid-imide containing compound. Similarly, U.S. Pat. No. 3,979,393 relates to the use of anhydride containing imidyl and isoimidyl compounds as curing agents for polyepoxide resins. In these examples, the imide containing adjuvants are low molecular weight materials which are primarily utilized as polyepoxide hardeners. Therefore, the quantity of imide containing adjuvant blended with the epoxy resin rarely exceeds that amount which is necessary to cure the epoxy resin.
The reason the imides are added as hardeners is that until now it has not been possible to blend or alloy polyimide resins in any quantity with epoxy resins to produce compatible resin blends suitable for laminating applications. For the most part, fully imidized polymers are insoluble in organic solvents, and, while some partially imidized polymers are soluble in organic solvents, the polyimide component is often not soluble in the epoxy component so that a compatible resin blend cannot be formed. In those cases where a blend of epoxy and polyimide resins can be formed which is compatible, very large amounts of organic solvent are required to achieve a blend solution viscosity which is suitable for the manufacture of prepregs. The necessity of evaporating and recovering these large amounts of solvents renders the process uneconomical.